This invention relates to laboratory apparatus useful in the assay of biological and biochemical reactants and is particularly concerned with multiwell apparatus capable of performing a separation process such as ultrafiltration, dialysis or reverse osmosis on a sample.
Ultrafiltration, dialysis and reverse osmosis processes all utilize a membrane to effect separation of component pars of a sample. In dialysis, for example, a sample is processed or purified by placing the sample against one surface of a semipermeable membrane and a pure solvent or solvent/solute dialysate mixture against the opposing surface of the membrane. The molecular weight cut-off (MWCO) of the membrane is selected to retain the sample, yet allow solutes having a smaller molecular size to pass through the membrane. The small molecular size solutes then diffuse throughout the membrane, driven by the concentration gradient of the small molecular size solute across the membrane. Separation of solutes in the sample is achieved by this mechanism.
In practice, a considerable part of the difficulty in performing dialysis lies in isolating the surface in contact with the sample from the opposing surface of the membrane. Cellulose acetate and regenerated cellulose dialysis membranes are notoriously difficult to seal thermally or with adhesives. It is present common practice to tie knots in tubings formed of these polymeric compositions or to utilize pinch clips in order to contain the sample.
Others clamp a flat membrane sheet between rigid plates, each containing individual receptacles. Samples are placed in the receptacles of the top plate and dialysate is circulated past the bottom plate. O-rings are utilized to seal receptacles in the top plate from one another. When utilizing this apparatus, it is difficult to attain uniform and effective pressure at the individual O-rings since pressure is not directly exerted on the individual O-rings. This lack of effective pressure application can cause undesirable across-talk between the sample receptacles. Also, since the bottom plate must press the membrane against the entire perimeter of each O-ring, the bottom receptacles form pockets dead-ended against the membrane. Air often is trapped in these pockets and prevents solvent from contacting the membrane. Even when solvent fills each pocket, dialysis is impeded because the dialysate in the pockets does not mix well with the bulk solvent.
Accordingly, it would be desirable to provide an apparatus for simultaneously dialyzing a plurality of samples while avoiding contamination between samples. In addition, it would be desirable to provide such an apparatus which assures direct contact of a separations membrane with a liquid such as a dialysate.